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Discussion Starter · #1 ·
Hi everyone. Looking for some information on the types of differentials used on various Subaru’s, obviously including the ascent. I saw this video mod:



I don’t think I’ll be doing anything like this video anytime soon but it did get me thinking if this was even possible with the ascent. What’s interesting to me is that some electrical ECU wiring makes it possible to have a switch to manually control the center diff. As opposed to letting the ECU control it. Ie low traction front wheels spins, takes a second to move MORE power all the while axles and gears are spinning. Center diff kicks in at speed obviously putting stress on diff gears. Hence why those who hoon out in the mud snow and dirt a lot can burn their diffs out. Seems better to be able to lock it while stationary first if you know you are in a low traction situation.
That said with the increase in the computer slip detection and VDC system I’m wondering if it would interfere? Or interfere with X-mode?

I assume that the rear differential is the “open” type but with VDC to help mitigate some wheel slip. Is the VDC better or preferred or is an after market limited slip rear diff theoretically going to out perform?
 

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Discussion Starter · #3 ·
Front and rear differentials are open.

The STI, uses both VDC and LSD's at the front and rear axles.

Electronics are a lot cheaper than mechanical LSD's.
are you aware if this electronic mod is possible on the ascent?

will this affect the VDC and traction control systems?
 

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are you aware if this electronic mod is possible on the ascent?

will this affect the VDC and traction control systems?
Nope, but if someone has the wiring diagrams, they can try it, at risk of seriously voiding the warranty.

Locking the clutch at 50:50 would not affect VDC and traction control systems, as other manufacturers that also use a electronic multi-plate clutch systems (usually those heavily FWD-biased systems) have a AWD lock button to fully lock the clutch to 50:50 up to 25 mph.
 

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Nope, but if someone has the wiring diagrams, they can try it, at risk of seriously voiding the warranty.

Locking the clutch at 50:50 would not affect VDC and traction control systems, as other manufacturers that also use a electronic multi-plate clutch systems (usually those heavily FWD-biased systems) have a AWD lock button to fully lock the clutch to 50:50 up to 25 mph.
Keep in mind that those systems don't effectively lock the system at 50:50 or apparently anywhere close. For instance, when the Palisade did a horrible job doing so during an off roading review, Hyundai responded with nice excuses explaining why their system couldn't do what it is marketed to do.

As for mechanical vs electronic, the mechanical systems in the OB and Crosstrek MT series are both very cheap and controlled electronically (via ABS instead of clutch control).

Both the electronically controlled not-a-center-differential in the Subaru ATS system and the mechanical not-a-center-differential viscous couplings in the former OB MT and in the Crosstrek MT vehicles are multiplate clutches.

I've been told that the real reason is gas mileage. Even with the better efficiency of the Subaru Symmetrical AWD system (less right angles and torque imbalance), implementing the easier slippable 60/40 ATs system is a way of scraping up a bit better gas mileage than a physically locked system.

As for affecting the VDC system, locking difs maybe possibly would, because we have active torque steering up to certain speeds. It would be interesting to see how the system handled incongruent data points for steering angle, wheel speeds, etc.
 

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Discussion Starter · #6 ·
Keep in mind that those systems don't effectively lock the system at 50:50 or apparently anywhere close. For instance, when the Palisade did a horrible job doing so during an off roading review, Hyundai responded with nice excuses explaining why their system couldn't do what it is marketed to do.

As for mechanical vs electronic, the mechanical systems in the OB and Crosstrek MT series are both very cheap and controlled electronically (via ABS instead of clutch control).

Both the electronically controlled not-a-center-differential in the Subaru ATS system and the mechanical not-a-center-differential viscous couplings in the former OB MT and in the Crosstrek MT vehicles are multiplate clutches.

I've been told that the real reason is gas mileage. Even with the better efficiency of the Subaru Symmetrical AWD system (less right angles and torque imbalance), implementing the easier slippable 60/40 ATs system is a way of scraping up a bit better gas mileage than a physically locked system.

As for affecting the VDC system, locking difs maybe possibly would, because we have active torque steering up to certain speeds. It would be interesting to see how the systejm handled incongruent data points for steering angle, wheel speeds, etc.
That is sort of my question. Is the ascent’s center diff similar to say the palisade? Or how does it operate differently? And when the palisades (poor) 50/50 split driving mode is called for it actually doesn’t work that well? Why?

in the video I posted there was a solenoid activation and deactivation which when engaged appeared to lock the multi plate clutch pack. Effectively making it a mechanical 4 wheel drive. Albeit without the limited slip front/rear. Robert is this what Xmode effectively does to the AWD system? Making this mod moot?

two questions still remain:
1) is this theoretically possible to hack the ECU/TCU explicitly (without installing anything Mechanical in the diff)
2) is there any benefit off road/snow above what XMode and VDC offer?
 

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As I understand it, the Ascent doesn't have a center diff. It has open front and rear diffs and a transfer case with a controllable clutch going to the rear. This is the simplest and least sophisticated mechanical system possible. It works well because a computer controls the the transfer clutch and applies brakes on spinning wheels. In contrast, my '91 Audi Quattro had an open front diff, a Torsen center diff and a lockable rear diff. My 97 AWD Explorer had a center diff with viscous coupling. There were no computers to control this and they worked great. The Ascent replaces mechanical complexity (some might say elegance) with (some might say "brute force") digital electronics. In the end, the Ascent's system and others like it are probably superior, although I'd prefer to have a center diff with computer controlled clutch so it wouldn't be so sensitive to having different dia tires in the front and rear.

I agree that locking having the ability to manually lock the Ascent's center diff could be useful at times.
 

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Discussion Starter · #8 ·
As I understand it, the Ascent doesn't have a center diff. It has open front and rear diffs and a transfer case with a controllable clutch going to the rear. This is the simplest and least sophisticated mechanical system possible. It works well because a computer controls the the transfer clutch and applies brakes on spinning wheels. In contrast, my '91 Audi Quattro had an open front diff, a Torsen center diff and a lockable rear diff. My 97 AWD Explorer had a center diff with viscous coupling. There were no computers to control this and they worked great. The Ascent replaces mechanical complexity (some might say elegance) with (some might say "brute force") digital electronics. In the end, the Ascent's system and others like it are probably superior, although I'd prefer to have a center diff with computer controlled clutch so it wouldn't be so sensitive to having different dia tires in the front and rear.

I agree that locking having the ability to manually lock the Ascent's center diff could be useful at times.
If I understand you correctly, you are saying that the transfer of power from front to back in our ascent is actually with a transfer case with a clutch (which I assume is electronically controlled). I'm not quite certain how this is different than a differential. If you could provide more insight that would be appreciated.

Additionally, if that is the case, is it not possible to hack in the TCU wiring to lock the clutch pack in the transfer case? Is this effectively what one of the things X-mode does (in addition to throttle, braking etc) or is it different?

If anyone has experiencing swapping out front/rear LSD's I'd love to hear from them, whether it makes sense to do. I'd love to put an LSD in the ascent's rear. I think I found one compatible with the R160 1 Bolt design of our open rear diff. This one:

 

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Replacing the rear is easy. Replacing the front requires splitting the CVT from the engine, and replacing the front differential in the bell housing casing. It's a somewhat integrated unit inside of the transmission's three body structures:
  • Bell housing/front diff
  • CVT
  • Rear differential in the nose cap
 

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... transfer of power from front to back in our ascent is actually with a transfer case with a clutch (which I assume is electronically controlled). I'm not quite certain how this is different than a differential...
A differential is a system of gears that transers power from a single input to two outputs while allowing for "differential" rotation of the two output shafts. Lot's of vids and explanations on youtube etc.

The Ascent drives the front wheels through gears and has an electronically controlled multi-plate clutch that drives the rear wheels. The transfer of power between the front and rear wheels is via the friction of the clutch, not through a geared differential.

Open differentials will spin the wheel with least traction (slipping) at the expense of the wheel with more traction. More complicated differentials, AKA limited slip differentials, have internal automatic mechanical mechanisms that limit this "slip". The Torsen differential is a very clever unique design that is inherently limited slip requiring no additional mechanisms to achieve that.

The Ascent has open differentials between the front wheels and rear wheels and controls the slip by applying the brakes on the slipping wheel.
 

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Discussion Starter · #11 ·
Keep in mind that those systems don't effectively lock the system at 50:50 or apparently anywhere close. For instance, when the Palisade did a horrible job doing so during an off roading review, Hyundai responded with nice excuses explaining why their system couldn't do what it is marketed to do.

As for mechanical vs electronic, the mechanical systems in the OB and Crosstrek MT series are both very cheap and controlled electronically (via ABS instead of clutch control).

Both the electronically controlled not-a-center-differential in the Subaru ATS system and the mechanical not-a-center-differential viscous couplings in the former OB MT and in the Crosstrek MT vehicles are multiplate clutches.

I've been told that the real reason is gas mileage. Even with the better efficiency of the Subaru Symmetrical AWD system (less right angles and torque imbalance), implementing the easier slippable 60/40 ATs system is a way of scraping up a bit better gas mileage than a physically locked system.

As for affecting the VDC system, locking difs maybe possibly would, because we have active torque steering up to certain speeds. It would be interesting to see how the system handled incongruent data points for steering angle, wheel speeds, etc.
Robert I still haven't been able to find the information regarding the center differential for our Ascent. I have learned a great deal about our R160 1-bolt open rear diff though.

What is the design of the 'gears'? Is it a transfer case? does it have planetary gears? multi-plate clutch?

Does it have a controllable hydraulic solenoid?
 

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Robert I still haven't been able to find the information regarding the center differential for our Ascent. I have learned a great deal about our R160 1-bolt open rear diff though.

What is the design of the 'gears'? Is it a transfer case? does it have planetary gears? multi-plate clutch?

Does it have a controllable hydraulic solenoid?
25 plate transfer clutch.
 

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Reviving an old thread. Anyone thought about this locking center diff mod or rear LSD diff install?
So, I was digging, and we have a different center differential than the TR580, made up of three clutch assemblies totaling around 40 plates.

I am wondering about the WRX center diff...
 

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Discussion Starter · #15 ·
Do you have or would be willing to share any technical documents relating to it. I am considering reading through it and if it's possible bring it to a trusted mechanic to help me create a center diff/clutch lock. Our Ascent is a 40 plate three clutch system? Earlier you mentioned it was a 25 plate (single?) clutch? The TCU wiring diagrams are also likely needed and very complicated haha.

I know X-mode adds 25% more clutch pressure, along with Engine, Transmission, and ABS input. I would like a switch to completely lock front/rear at 50/50.
Question:
What is the limit of clutch pressure? is 25% increase a max? ie is it actually locked?
 

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Do you have or would be willing to share any technical documents relating to it. I am considering reading through it and if it's possible bring it to a trusted mechanic to help me create a center diff/clutch lock. Our Ascent is a 40 plate three clutch system? Earlier you mentioned it was a 25 plate (single?) clutch? The TCU wiring diagrams are also likely needed and very complicated haha.

I know X-mode adds 25% more clutch pressure, along with Engine, Transmission, and ABS input. I would like a switch to completely lock front/rear at 50/50.
Question:
What is the limit of clutch pressure? is 25% increase a max? ie is it actually locked?
Yep, as I said, I did some more digging, and found that the TR690 apparently uses three clutches. My guess is two of them are engaged whenever the car is in drive. So, the quantity of plates engaged is pretty similar, but the design is drastically different.

The TR580 has a forward and reverse gear before the pulley cones (the variators). That means that the pulleys and chain need to stop and reverse direction when the car moves from forward to reverse (and vice versa, of course). The chain and pulleys do not spin when the car is in park or neutral.

The TR690 always spins the chains and pulleys, and always spins them the same direction. The forward/reverse mechanism is thus after the variator pulley cones on the drive side, instead of on the driven side.

There's a doc someplace online that shows the assembly... I will see if I can find it.
 

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Isn't the Ascent's center clutch under control of the AWD system such that it varies clutch engagement pressure to control f/r torque proportioning? If so, theoretically, it should be possible to effectively lock it via software, or maybe with an added override switch.
 

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Isn't the Ascent's center clutch under control of the AWD system such that it varies clutch engagement pressure to control f/r torque proportioning? If so, theoretically, it should be possible to effectively lock it via software, or maybe with an added override switch.
Yes, that's what X-Mode does, but, X-Mode turns off at 20mph.

X-Mode, on the Ascent, also changes the low "gear" ratio.
 
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